2002 Lancer: oil light started coming on at idle..changed oil

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2002 Lancer OZ - oil light started coming on at idle. Changed switch and changed oil. Went away for a while. Now light back on. First came on below 500 RPM, then started coming on below 1000 rpm, last night started coming on below 1500 rpm.Is the pump shot? Any other possibilities other than pump?